Gas Gauge Troubleshooting (early model)

[cabin]

So ever since I bought my car, the fuel gage has been inoperable. The car is an 85 325e. With the car running or even in ACC position, the gauge will read as being full. Of course when off, the gauge will go to the empty position.

When the fuel level is getting low, the warning light will illuminate, even though the needle is pegged at full. The OBC will also read as full.

Now I haven't extensively troubleshot this, but here is what has been done so far:
-removed cluster, found burnt trace going to gas gauge. Soldered in a jumper wire, NO CHANGE
-cleaned any corrosion off nut and mating surface on back of cluster, NO CHANGE
-tried hitting dash to no avail...

Here is where I made some progress, but it leaves me stumped,
-removed fuel sender from tank and cleaned with electric contact cleaner
-cleaned plug and receptacle with contact cleaner

After doing this, I turned the car on and the gauge gave me a legitimate reading. This lasted seconds and it shot back to full. A few days later, I went back out and unplugged the sender again and plugged it back in and the gauge worked again, but only for a few seconds...

Now all the wiring appears to be in good condition as well as the connector and sender. Where could this problem lie?

[jlevie]

The gauge reading full all of the time is short in either the wiring or the sensor. If the gauge goes to zero when you disconnect the sensor, the sensor is at fault.

[cabin]

Thanks for the reply jlevie.

What explains the gauge working for a short amount of time when disconnected then reconnected? That is what is throwing me off here. Maybe the sender is shorted internally?

[Gregs///M]

Unplug connector at sender and see if if the gauge drops. If so, short is in the sender. If not, could be the wiring up to the cluster.